Antibodies specific for the purine and pyrimidine bases of nucleic acids may be obtained using nucleoside- or nucleotide-protein conjugates. Such antibodies react with denatured DNA and with RNA. The objectives of the proposed research are to delineate the utility of these antibodies for immunochemical and biochemical investigations of nucleic acids. Among the studies proposed are: a) the use of antibodies to distinguish among DNA's of varying base composition. Some evidence has been obtained that this is possible for bacterial DNA's. b) the use of antibodies to fractionate DNA. Evidence has been obtained that anti-T precipitates a fraction richer in thymine than the original DNA. c) attempts to analyze DNA immunochemically for minor bases. Antibodies specific for minor bases can be obtained. d) the use of antibodies to distinguish among the various types of RNA. e) investigation of the effects of anti-nucleic acids on cells in tissue culture. f) analysis of the determinant groups of tRNA of known sequence. g) elucidation of the biochemical basis for the inhibition of the embryonic development of the fertilized sea urchin egg by purine- and pyrimidine-specific antibodies.